Do bacteria influence the sensitivity of phytoplankton to allelochemicals from submerged macrophytes?

摘要

The inhibition of phytoplankton by allelochemicals releasedfrom submerged macrophytes has been demonstrated in numerous studies, but the relevance of this process at ecosystem level is still debated because in situ evidence is difficult to achieve (GROSS et al. 2007, HILT & GROSS 2008). An unsolved question is whether microorganisms interfere with this process by transforming allelopathically active substances (GROSS et al. 1996, NAKA I et al, 1999) or otherwise modifying the effect of allelochemicals on phytoplankton. Bacteria suspended in the surrounding water as well as those in bioftlms attached to the exuding submerged macrophytes and the target phytoplankton species may potentially have an impact. MULLER et al. (2007) were the first to isolate bacteria associated with the surrounding water column and the attached biofilm of an allelopathically active submerged macrophyte species (Myriophyllum spicatum L.) that were able to degrade polyphenols, the main allelopathically active compounds. The presence of polyphenoldegrading bacteria strains in the plant's biofilm suggested a selection effect of M. spicatum exudates in favour of highly specialized bacteria and explains the rapid disappearance of specific M. spicatum polyphenols after excretion (MOLLER et al. 2007). The same polyphenols exerted some antimicrobial activity (WALENCIAK et al. 2002). Both antimicrobial effects and the presence of specific polyphenol degrading bacteria may affect the sensitivity of phytoplankton to specific allelochemicals because individual algal species are characterized by specific bacterial communities (GROSSART et al. 2005). Bacteria- algae interactions range from stimulation (supply of limiting nutrients and vitamins by heterotrophic remineralization) over inactivation of toxic or inhibiting substances to negative interactions (e.g., competition for limiting nutrients or by modification of the microenvironment; COLE 1982). Whether specific bacteria indeed contribute to the sensitivity of algae to allelochemicals is yet unknown. We compared the effect of tannic acid (TA) on the growth of axenic and xenic cultures of the green alga Desmodesmus armatus Chodat to test whether the presence of bacteria has a significant influence on the sensitivity of the algae to this allelopathically active compound.